Many of the pathologies of the heart lead to a deterioration in its ability to pump blood. In this situation, the heart itself and the organs of the body are at great risk for sustaining irreversible damage. Ventricular assist devices can be implanted to ensure that the organs of the body are supplied with an adequate flow of blood by taking over the pumping action of the heart. Ventricular assist devices can help heart transplant patients survive until a suitable donor heart is found. They can also help the heart return to normal function after heart surgery. Or they can be used as permanent devices in cases of severe heart failure where heart transplantation is not a viable option for the patient.
Most commonly used ventricular assist devices are pumping devices, which shunt blood from the left ventricle of the heart to the aorta. Use of a ventricular assist pump was shown to help promote improved heart function in some patients with heart failure. The most significant problem that patients with ventricular assist pumps face stems from the fact that the patient's blood is constantly circulated through the man-made surfaces of the device. This frequent contact with the surface of the device increases the likelihood that components of the patient's blood will react to the presence of a foreign object and result in blood clots, infections, and immune system reactions.
Another type of ventricular assist devices consists of an assembly that serves to apply pressure to the surface of the heart in order to augment or replace its pumping action. This type of device has the advantage of minimal contact with the patient's blood, and so the risk of blood clots, infections and immune system reactions is significantly reduced. The drawback of this type of device is that repeated cycling of pressure to the exterior of the heart can, lead to mechanical trauma to the surface of the heart. In addition, these types of devices do not localize the pressure to the damaged area of the heart but also apply pressure to healthy areas of the heart.
Thus there is a need to supply mechanical energy in a localized fashion to the non-functional portions of the heart muscle, thereby minimizing the potential to traumatize otherwise healthy portions of the heart, as well as minimizing the total energy required to restore the patient's heart function.
“Smart skin” or artificial muscle could in theory be used to cover the nonfunctional area of the heart and restore functionality. However, a challenge in implementing this approach is the ability to distribute sufficient energy and power in the artificial muscle, especially on the relatively small scales of interest, since in many cases the nonfunctional areas of the heart are on the order of only tens of millimeters in diameter.
Any assist device must have sufficient energy and power density in order for it to perform the work necessary to aid in circulating blood. A critical issue is whether the energy or power can be distributed in an appropriate fashion throughout the volume of the device, especially as the total physical size of the device scales down. Although electrical energy can be easily dispersed throughout the volume of a ventricular assist device, conversion of this electrical energy to mechanical energy by means of a multiplicity of small-scale-localized actuators is difficult, due to the unfavorable scaling of many of these actuators. Magnetic and electrostatic actuators cannot supply sufficient energy density for this type of device on the small scale. Piezoelectric actuators can supply the required energy density but must utilize very high electric potentials, a condition that is not ideal for biocompatibility. Shape memory alloys can also supply the required energy density but require heating and cooling cycling which would not be good for the heart, and in addition may not be able to withstand the cycling of the heart without undergoing fatigue-based failure. Thus none of these types of actuating mechanisms are well suited to the demands of a ventricular assist device.
Still another consideration in the design of a ventricular assist device is the fact that, if the device were to fail, the presence of the failed ventricular assist device should not increase the demands on a partially dysfunctional heart. If the device fails and the heart has to pump against the failed device, heart failure is rendered more likely. Thus a desirable characteristic of a ventricular assist device is that, if it were to fail, it should place a neutral demand on the unassisted heart and so not contribute to adverse clinical consequences.